When dedupe wants to use the page cache to compare parts of two files
for dedupe, we must be very careful to handle locking correctly.  The
current code doesn't do this.  It must lock and unlock the page only
once if the two pages are the same, since the overlapping range check
doesn't catch this when blocksize < pagesize.  If the pages are distinct
but from the same file, we must observe page locking order and lock them
in order of increasing offset to avoid clashing with writeback locking.

Fixes: 876bec6f ("vfs: refactor clone/dedupe_file_range common functions")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>