Documentation: ext4: Add fields to ext4_super_block documentation

Documentation: ext4: Add fields to ext4_super_block documentation

Documentation and implementation of the ext4 super block have
slightly diverged: Padding has been removed in order to make room for
new fields that are still missing in the documentation.

Add the new fields s_encryption_level, s_first_error_errorcode,
s_last_error_errorcode to the documentation of the ext4 super block.

Fixes: f542fbe8d5e8 ("ext4 crypto: reserve codepoints used by the ext4 encryption feature")
Fixes: 878520ac45f9 ("ext4: save the error code which triggered an ext4_error() in the superblock")

Signed-off-by: Tom Vierjahn <tom.vierjahn@acm.org>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/20250324221004.5268-1-tom.vierjahn@acm.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>