mm/swapfile.c: fix a comment in sys_swapon()

ANBZ: torvalds#1075

commit fed98ef upstream.

claim_swapfile now always takes i_rwsem.

Link: http://lkml.kernel.org/r/20200114161225.309792-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Hongnan Li <hongnan.li@linux.alibaba.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>

mm/swapfile.c

@@ -3242,7 +3242,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	mapping = swap_file->f_mapping;
 	inode = mapping->host;
 
-	/* If S_ISREG(inode->i_mode) will do inode_lock(inode); */
+	/* will take i_rwsem; */
 	error = claim_swapfile(p, inode);
 	if (unlikely(error))
 		goto bad_swap;