xfs_repair: update the manual content about xfs_repair exit status

xfs_repair: update the manual content about xfs_repair exit status

The man 8 xfs_repair said "xfs_repair run without the -n option will
always return a status code of 0". That's not correct.

xfs_repair will return 2 if it finds a fs log which needs to be
replayed or cleared, 1 if runtime error is encountered, and 0 for
all other cases.

[ sandeen: editing for clarity ]

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>