ext4: fix error pointer dereference in ext4_mb_load_buddy_gfp()

This code calls folio_put() on an error pointer which will lead to a
crash.  Check for both error pointers and NULL pointers before calling
folio_put().

Fixes: 5eea586b

 ("ext4: convert bd_buddy_page to bd_buddy_folio")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/eaafa1d9-a61c-4af4-9f97-d3ad72c60200@moroto.mountain
Signed-off-by: Theodore Ts'o <tytso@mit.edu>