ext2: Drop GFP_NOFS use in ext2_get_blocks()

ext2_get_blocks() calls sb_issue_zeroout() with GFP_NOFS flag. However
the call is performed under inode->i_rwsem and
EXT2_I(inode)->i_truncate_mutex neither of which is acquired during
direct fs reclaim. So it is safe to change the gfp mask to GFP_KERNEL.

Signed-off-by: Jan Kara <jack@suse.cz>