nilfs2: prevent kernel bug at submit_bh_wbc()

Fix a bug where nilfs_get_block() returns a successful status when
searching and inserting the specified block both fail inconsistently.  If
this inconsistent behavior is not due to a previously fixed bug, then an
unexpected race is occurring, so return a temporary error -EAGAIN instead.

This prevents callers such as __block_write_begin_int() from requesting a
read into a buffer that is not mapped, which would cause the BUG_ON check
for the BH_Mapped flag in submit_bh_wbc() to fail.

Link: https://lkml.kernel.org/r/20240313105827.5296-3-konishi.ryusuke@gmail.com
Fixes: 1f5abe7 ("nilfs2: replace BUG_ON and BUG calls triggerable from ioctl")
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: konis

fs/nilfs2/inode.c

@@ -112,7 +112,7 @@ int nilfs_get_block(struct inode *inode, sector_t blkoff,
 					   "%s (ino=%lu): a race condition while inserting a data block at offset=%llu",
 					   __func__, inode->i_ino,
 					   (unsigned long long)blkoff);
-				err = 0;
+				err = -EAGAIN;
 			}
 			nilfs_transaction_abort(inode->i_sb);
 			goto out;