ext4: remove ext4_new_blocks() and call ext4_mb_new_blocks() directly

There was only one caller of the compatibility function
ext4_new_blocks(), in balloc.c's ext4_alloc_blocks().  Change it to
call ext4_mb_new_blocks() directly, and remove ext4_new_blocks()
altogether.  This cleans up the code, by removing two extra functions
from the call chain, and hopefully saving some stack usage.

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