xfs_io: don't count fsmaps before querying fsmaps

xfs_io: don't count fsmaps before querying fsmaps

There's a bunch of code in fsmap.c that tries to count the GETFSMAP
records so that it can size the fsmap array appropriately for the
GETFSMAP call.  It's pointless to iterate the entire result set /twice/
(unlike the bmap command where the extent count is actually stored in
the fs metadata), so get rid of the duplicate walk.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>