xfs: make iomap_begin functions trim iomaps consistently

Historically, the XFS iomap_begin function only returned mappings for
exactly the range queried, i.e. it doesn't do XFS_BMAPI_ENTIRE lookups.
The current vfs iomap consumers are only set up to deal with trimmed
mappings.  xfs_xattr_iomap_begin does BMAPI_ENTIRE lookups, which is
inconsistent with the current iomap usage.  Remove the flag so that both
iomap_begin functions behave the same way.

FWIW this also fixes a behavioral regression in xattr FIEMAP that was
introduced in 4.8 wherein attr fork extents are no longer trimmed like
they used to be.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>