ext4: make ext4_mb_initialize_context return void

Change the return type to void since it always return 0, and no need
to do the checking in ext4_mb_new_blocks.

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20221202120409.24098-1-guoqing.jiang@linux.dev
Signed-off-by: Theodore Ts'o <tytso@mit.edu>