ext4: factor out ext4_map_create_blocks() to allocate new blocks

Factor out a common helper ext4_map_create_blocks() from
ext4_map_blocks() to do a real blocks allocation, no logic changes.

[ Note: this first patch of a ten patch series named "v3: simplify the
  counting and management of delalloc reserved blocks".  The link to
  the v1 and v2 patch series are below. -- TYT ]

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240802115120.362902-1-yi.zhang@huaweicloud.com # v2 of patch series
Link: https://patch.msgid.link/20240601034149.2169771-1-yi.zhang@huaweicloud.com # v1 of the patch series

Link: https://patch.msgid.link/20240813123452.2824659-2-yi.zhang@huaweicloud.com


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