btrfs: introduce btrfs_inode_lock()/unlock()

btrfs_inode_lock/unlock() are wrappers around inode locks, separating
the type of lock and actual locking.

- 0 - default, exclusive lock
- BTRFS_ILOCK_SHARED - for shared locks, for possible parallel DIO
- BTRFS_ILOCK_TRY - for the RWF_NOWAIT sequence

The bits SHARED and TRY can be combined together.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>