Out of curiosity, why does btrfs_update_time() need to call
btrfs_root_readonly()?  Why can't it just depend on the
__mnt_want_write() call in touch_atime()?

Surely if there are times when it's not OK to write into a btrfs file
system and mnt_is_readonly() returns false, the VFS is going to get
very confused abyway.

If the btrfs_update_time() is not necessary, then we could drop
btrfs_update_time() and update_time() from the inode operations
entirely, and depend on the VFS-level code in update_time().

	      	     	    	      	   - Ted