ext4: factor out loop for freeing inode xattr space

Move loop to make enough space in the inode from
ext4_expand_extra_isize_ea() into a separate function to make that
function smaller and better readable and also to avoid delaration of
variables inside a loop block.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>