GFS2: Use inode_newsize_ok and get_write_access in fallocate

gfs2_fallocate wasn't checking inode_newsize_ok nor get_write_access.
Split out the context setup and inode locking pieces into a separate
function to make it more clear and add these missing calls.

inode_newsize_ok is called conditional on FALLOC_FL_KEEP_SIZE as there
is no need to enforce a file size limit if it isn't going to change.

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>