mkfs: fix libxfs_iget return value sign inversion

mkfs: fix libxfs_iget return value sign inversion

libxfs functions return negative errno, so utilities must invert the
return values from such functions.  Caught by xfs/437.

Fixes: 8a4ea72724930c ("proto: add ability to populate a filesystem from a directory")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>