libxfs: split new inode creation into two pieces

libxfs: split new inode creation into two pieces

Source kernel commit: 38fd3d6a956f1b104f11cd6eee116c54bfe458c4

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>