ext4: factor out xattr moving

Factor out function for moving xattrs from inode into external xattr
block from ext4_expand_extra_isize_ea(). That function is already quite
long and factoring out this rather standalone functionality helps
readability.

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