libxfs: remove libxfs_dir_ialloc

libxfs: remove libxfs_dir_ialloc

This function no longer exists in the kernel, and it's not really needed
in userspace either.  There are two users of it: repair and mkfs.
xfs_repair and xfs_db do not have useful cred and fsxattr structures so
they can call libxfs_dialloc and libxfs_icreate directly.  For mkfs
we'll move the guts of libxfs_dir_ialloc into proto.c as a creatproto
function that handles setting user/group ids, and move struct cred to
mkfs since it's now the only user.

This gets us ready to hoist the rest of the inode initialization code to
libxfs for metadata directories.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>