Documentation: Correct s_umount state for freeze_fs/unfreeze_fs

freeze_fs/unfreeze_fs ops are called with s_umount held for write, not read.
Signed-off-by: NValerie Aurora <val@vaaconsulting.com>
Signed-off-by: NKamal Mostafa <kamal@canonical.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/Locking

@@ -138,8 +138,8 @@ evict_inode:
 put_super:		write
 write_super:		read
 sync_fs:		read
-freeze_fs:		read
-unfreeze_fs:		read
+freeze_fs:		write
+unfreeze_fs:		write
 statfs:			maybe(read)	(see below)
 remount_fs:		write
 umount_begin:		no