lib: bitmap.c: get rid of some doc warnings

There are two ascii art drawings there. Use a block markup tag there
in order to get rid of those warnings:

	./lib/bitmap.c:189: WARNING: Unexpected indentation.
	./lib/bitmap.c:190: WARNING: Block quote ends without a blank line; unexpected unindent.
	./lib/bitmap.c:190: WARNING: Unexpected indentation.
	./lib/bitmap.c:191: WARNING: Line block ends without a blank line.

It should be noticed that there's actually a syntax violation
right now, as something like:

	/**
	 ...
	 @src:

will be handled as a definition for @src parameter, and not as
part of a diagram. So, we need to add something before it, in
order for this to be processed the way it should.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1e2568fdfa838c1a0d8cc2a1d70dd4b6de99bfb1.1586881715.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

lib/bitmap.c

@@ -182,21 +182,22 @@ EXPORT_SYMBOL(__bitmap_shift_left);
  *
  * In pictures, example for a big-endian 32-bit architecture:
  *
- * @src:
- * 31                                   63
- * |                                    |
- * 10000000 11000001 11110010 00010101  10000000 11000001 01110010 00010101
- *                 |  |              |                                    |
- *                16  14             0                                   32
- *
- * if @cut is 3, and @first is 14, bits 14-16 in @src are cut and @dst is:
- *
- * 31                                   63
- * |                                    |
- * 10110000 00011000 00110010 00010101  00010000 00011000 00101110 01000010
- *                    |              |                                    |
- *                    14 (bit 17     0                                   32
- *                        from @src)
+ * The @src bitmap is::
+ *
+ *   31                                   63
+ *   |                                    |
+ *   10000000 11000001 11110010 00010101  10000000 11000001 01110010 00010101
+ *                   |  |              |                                    |
+ *                  16  14             0                                   32
+ *
+ * if @cut is 3, and @first is 14, bits 14-16 in @src are cut and @dst is::
+ *
+ *   31                                   63
+ *   |                                    |
+ *   10110000 00011000 00110010 00010101  00010000 00011000 00101110 01000010
+ *                      |              |                                    |
+ *                      14 (bit 17     0                                   32
+ *                          from @src)
  *
  * Note that @dst and @src might overlap partially or entirely.
  *