nilfs2: remove individual gfp constants for each metadata file

This gets rid of NILFS_CPFILE_GFP, NILFS_SUFILE_GFP, NILFS_DAT_GFP,
and NILFS_IFILE_GFP.  All of these constants refer to NILFS_MDT_GFP,
and can be removed.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>