fix up ext2_fs.h for userspace after reservations backport

In commit a686cd89:

 "Val's cross-port of the ext3 reservations code into ext2."

include/linux/ext2_fs.h got a new function whose return value is only
defined if __KERNEL__ is defined. Putting #ifdef __KERNEL__ around the
function seems to help, patch below.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext2/ext2.h

@@ -178,3 +178,10 @@ extern const struct inode_operations ext2_special_inode_operations;
 /* symlink.c */
 extern const struct inode_operations ext2_fast_symlink_inode_operations;
 extern const struct inode_operations ext2_symlink_inode_operations;
+
+static inline ext2_fsblk_t
+ext2_group_first_block_no(struct super_block *sb, unsigned long group_no)
+{
+	return group_no * (ext2_fsblk_t)EXT2_BLOCKS_PER_GROUP(sb) +
+		le32_to_cpu(EXT2_SB(sb)->s_es->s_first_data_block);
+}

include/linux/ext2_fs.h

@@ -563,11 +563,4 @@ enum {
 					 ~EXT2_DIR_ROUND)
 #define EXT2_MAX_REC_LEN		((1<<16)-1)
 
-static inline ext2_fsblk_t
-ext2_group_first_block_no(struct super_block *sb, unsigned long group_no)
-{
-	return group_no * (ext2_fsblk_t)EXT2_BLOCKS_PER_GROUP(sb) +
-		le32_to_cpu(EXT2_SB(sb)->s_es->s_first_data_block);
-}
-
 #endif	/* _LINUX_EXT2_FS_H */