In key rotation, we must initialize unallocated but previously
initialized pages, so that if encryption is enabled on a table,
all clear-text data for the page will eventually be overwritten.
But we should not rotate keys on pages that were never allocated
after the data file was created.

According to the latching order rules, after acquiring the
tablespace latch, no page latches of previously allocated user pages
may be acquired. So, key rotation should check the page allocation
status after acquiring the page latch, not before. But, the latching
order rules also prohibit accessing pages that were not allocated first,
and then acquiring the tablespace latch. Such behaviour would indeed
result in a deadlock when running the following tests:
encryption.innodb_encryption-page-compression
encryption.innodb-checksum-algorithm

Because the key rotation is accessing potentially unallocated pages, it
cannot reliably check if these pages were allocated. It can only check
the page header. If the page number is zero, we can assume that the
page is unallocated.

fil_crypt_rotate_page(): Detect uninitialized pages by FIL_PAGE_OFFSET.
Page 0 is never encrypted, and on other pages that are initialized,
FIL_PAGE_OFFSET must contain the page number.

fil_crypt_is_page_uninitialized(): Remove. It suffices to check the
page number field in fil_crypt_rotate_page().