ext4: remove do_blk_alloc()

The convenience function do_blk_alloc() is a static function with only
one caller, so fold it into ext4_new_meta_blocks() to simplify the
code and to make it easier to understand.

To save more stack space, if count is a null pointer in
ext4_new_meta_blocks() assume that caller wanted a single block (and
if there is an error, no blocks were allocated).

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