ext2: factor our freeing of xattr block reference

Free of xattr block reference is opencode in two places. Factor it out
into a separate function and use it.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220712105436.32204-6-jack@suse.cz


Signed-off-by: Theodore Ts'o <tytso@mit.edu>