mds: fix bug filelock stuck at LOCK_XSYN leading client can't read data

mds: fix bug filelock stuck at LOCK_XSYN leading client can't read data

The file's wanted_loner_cap will be -1 when more than one client open
it.Sometimes the loner can't be dropped successfully which will leading
filelock stuck at LOCK_XSYN.However,this state doesn't allow others any
caps so the client open the file by O_RDONLY can't read data all the time
untill someone else open it by O_RDWR or O_WRONLY.We should change the
state to LOCK_MIX or LOCK_SYNC instead of stucking all the time.

Fixes: http://tracker.ceph.com/issues/37333
Signed-off-by: Guan yunfei <yunfei.guan@xtaotech.com>