ext4: fold ext4_alloc_blocks() in ext4_alloc_branch()

The older code was far more complicated than it needed to be because
of how we spliced in the ext4's new multiblock allocator into ext3's
indirect block code.  By folding ext4_alloc_blocks() into
ext4_alloc_branch(), we make the code far more understable, shave off
over 130 lines of code and half a kilobyte of compiled object code.

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