xfs: add XFS_IOC_FREE_EOFBLOCKS ioctl

The XFS_IOC_FREE_EOFBLOCKS ioctl allows users to invoke an EOFBLOCKS
scan. The xfs_eofblocks structure is defined to support the command
parameters (scan mode).

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>