GFS2: Allocate reservation during splice_write

This patch adds a GFS2-specific function for splice_write which
first calls function gfs2_rs_alloc to make sure a reservation
structure has been allocated before attempting to reserve blocks.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>

diff --cc fs/gfs2/file.c
Simple merge