xfs: split new inode creation into two pieces

xfs: split new inode creation into two pieces

There are two parts to initializing a newly allocated inode: setting up
the incore structures, and initializing the new inode core based on the
parent inode and the current user's environment.  The initialization
code is not specific to the kernel, so we would like to share that with
userspace by hoisting it to libxfs.  Therefore, split xfs_icreate into
separate functions to prepare for the next few patches.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>