Factor out __es_alloc_extent() and __es_free_extent(), which only allocate
and free extent_status in these two helpers.

The ext4_es_alloc_extent() function is split into __es_alloc_extent()
and ext4_es_init_extent(). In __es_alloc_extent() we allocate memory using
GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO if the memory allocation cannot
fail, otherwise we use GFP_ATOMIC. and the ext4_es_init_extent() is used to
initialize extent_status and update related variables after a successful
allocation.

This is to prepare for the use of pre-allocated extent_status later.
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230424033846.4732-4-libaokun1@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>