btrfs: add missing mutex_unlock in btrfs_relocate_sys_chunks()

btrfs: add missing mutex_unlock in btrfs_relocate_sys_chunks()

The previous patch that replaced BUG_ON by error handling forgot to
unlock the mutex in the error path.

Link: https://lore.kernel.org/all/Zh%[email protected]
Reported-by: Pavel Machek <[email protected]>
Fixes: 7411055db5ce ("btrfs: handle chunk tree lookup error in btrfs_relocate_sys_chunks()")
CC: [email protected]
Reviewed-by: Pavel Machek <[email protected]>
Signed-off-by: Dominique Martinet <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>