xfs: improve xfs_forget_acl

Move the function to xfs_acl.c and provide a proper stub for the
!CONFIG_XFS_POSIX_ACL case.  Lift the flags check to the caller as it
nicely fits in there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Chandan Rajendra <chandanrlinux@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>