xfs: factor prid related codes into xfs_get_initial_prid()

It will be reused by the O_TMPFILE creation function.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --cc fs/xfs/xfs_inode.c
Simple merge

diff --cc fs/xfs/xfs_inode.h
Simple merge

diff --cc fs/xfs/xfs_symlink.c
Simple merge