libext2fs: provide a function to set inode size

libext2fs: provide a function to set inode size

Provide an API to set i_size in an inode and take care of all required
feature flag modifications.  Refactor the code to use this new
function.

[ Moved the function to lib/ext2fs/blk_num.c, which is the rest of
  these sorts of functions live, and renamed it to be
  ext2fs_inode_size_set() instead of ext2fs_inode_set_size() to be
  consistent with the other functions in in blk_num.c -- tytso ]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>