ext4: add EXT4_IOC_ALLOC_DA_BLKS ioctl

Add an ioctl which forces all of the delay allocated blocks to be
allocated.  This also provides a function ext4_alloc_da_blocks() which
will be used by the following commits to force files to be fully
allocated to preserve application-expected ext3 behaviour.

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