gfs2: Use gfs2 wrapper to sync inode before calling generic_file_splice_read()

gfs2_file_splice_read() f_op grabs and releases the cluster-wide
inode glock to sync the inode size to the latest.

Without this, generic_file_splice_read() uses an older i_size value
and can return EOF for valid offsets in the inode.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>