ext4: save the error code which triggered an ext4_error() in the superblock

This allows the cause of an ext4_error() report to be categorized
based on whether it was triggered due to an I/O error, or an memory
allocation error, or other possible causes.  Most errors are caused by
a detected file system inconsistency, so the default code stored in
the superblock will be EXT4_ERR_EFSCORRUPTED.

Link: https://lore.kernel.org/r/20191204032335.7683-1-tytso@mit.edu
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/extents.c

@@ -492,6 +492,7 @@ static int __ext4_ext_check(const char *function, unsigned int line,
 	return 0;
 
 corrupted:
+	ext4_set_errno(inode->i_sb, -err);
 	ext4_error_inode(inode, function, line, 0,
 			 "pblk %llu bad header/extent: %s - magic %x, "
 			 "entries %u, max %u(%u), depth %u(%u)",