UBIFS: inline one-line functions

'ubifs_get_lprops()' and 'ubifs_release_lprops()' basically wrap
mutex lock and unlock. We have them because we want lprops subsystem
be separate and as independent as possible. And we planned better
locking rules for lprops.

Anyway, because they are short, it is better to inline them.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>