CRED: Wrap task credential accesses in the FAT filesystem

Wrap access to task credentials so that they can be separated more easily from
the task_struct during the introduction of COW creds.

Change most current->(|e|s|fs)[ug]id to current_(|e|s|fs)[ug]id().

Change some task->e?[ug]id to task_e?[ug]id().  In some places it makes more
sense to use RCU directly rather than a convenient wrapper; these will be
addressed by later patches.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: James Morris <jmorris@namei.org>

fs/fat/file.c

@@ -302,7 +302,7 @@ static int fat_allow_set_time(struct msdos_sb_info *sbi, struct inode *inode)
 {
 	mode_t allow_utime = sbi->options.allow_utime;
 
-	if (current->fsuid != inode->i_uid) {
+	if (current_fsuid() != inode->i_uid) {
 		if (in_group_p(inode->i_gid))
 			allow_utime >>= 3;
 		if (allow_utime & MAY_WRITE)

fs/fat/inode.c

@@ -921,8 +921,8 @@ static int parse_options(char *options, int is_vfat, int silent, int *debug,
 
 	opts->isvfat = is_vfat;
 
-	opts->fs_uid = current->uid;
-	opts->fs_gid = current->gid;
+	opts->fs_uid = current_uid();
+	opts->fs_gid = current_gid();
 	opts->fs_fmask = opts->fs_dmask = current->fs->umask;
 	opts->allow_utime = -1;
 	opts->codepage = fat_default_codepage;