xfs: fix broken logic when detecting mergeable bmap records

Commit 6bc6c99a944c was a well-intentioned effort to initiate
consolidation of adjacent bmbt mapping records by setting the PREEN
flag.  Consolidation can only happen if the length of the combined
record doesn't overflow the 21-bit blockcount field of the bmbt
recordset.  Unfortunately, the length test is inverted, leading to it
triggering on data forks like these:

 EXT: FILE-OFFSET           BLOCK-RANGE        AG AG-OFFSET               TOTAL
   0: [0..16777207]:        76110848..92888055  0 (76110848..92888055) 16777208
   1: [16777208..20639743]: 92888056..96750591  0 (92888056..96750591)  38625368

Note that record 0 has a length of 16777208 512b blocks.  This
corresponds to 2097151 4k fsblocks, which is the maximum.  Hence the two
records cannot be merged.

However, the logic is still wrong even if we change the in-loop
comparison, because the scope of our examination isn't broad enough
inside the loop to detect mappings like this:

   0: [0..9]:               76110838..76110847  0 (76110838..76110847)       10
   1: [10..16777217]:       76110848..92888055  0 (76110848..92888055) 16777208
   2: [16777218..20639753]: 92888056..96750591  0 (92888056..96750591)  38625368

These three records could be merged into two, but one cannot determine
this purely from looking at records 0-1 or 1-2 in isolation.

Hoist the mergability detection outside the loop, and base its decision
making on whether or not a merged mapping could be expressed in fewer
bmbt records.  While we're at it, fix the incorrect return type of the
iter function.

Fixes: 336642f7 ("xfs: alert the user about data/attr fork mappings that could be merged")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>