fs: introduce bgl_lock_ptr()

As suggested by Andreas Dilger, introduce a bgl_lock_ptr() helper in
<linux/blockgroup_lock.h> and add separate sb_bgl_lock() helpers to
filesystem specific header files to break the hidden dependency to
struct ext[234]_sb_info.

Also, while at it, convert the macros to static inlines to try make up
for all the times I broke Andrew Morton's tree.

Acked-by: Andreas Dilger <adilger@sun.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ext4/ext4_sb.h

@@ -146,4 +146,10 @@ struct ext4_sb_info {
 	struct flex_groups *s_flex_groups;
 };
 
+static inline spinlock_t *
+sb_bgl_lock(struct ext4_sb_info *sbi, unsigned int block_group)
+{
+	return bgl_lock_ptr(&sbi->s_blockgroup_lock, block_group);
+}
+
 #endif	/* _EXT4_SB */