afs_fsync() calls filemap_write_and_wait_range() and then does a walk
through the writeback records and flushes those - which should achieve
exactly the same thing.

Get rid of the filemap_write_and_wait_range() since that's uninterruptible,
whereas the wait for the writeback records is interruptible.

Further, we can at least contract the inode-locked region to just the
afs_writeback_call().
Signed-off-by: David Howells <dhowells@redhat.com>