ext4: remove unneeded test in ext4_alloc_file_blocks()

ext4_alloc_file_blocks() is called from ext4_zero_range() and
ext4_fallocate() both already testing EXT4_INODE_EXTENTS
We can call ext_depth(inode) unconditionnally.

[ Added BUG_ON check to make sure ext4_alloc_file_blocks() won't get
  called for a indirect-mapped inode in the future.  -- tytso ]

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>