Ignore EEXIST error when creating a directory

On lola, we reproduced this persist 8003 error too and John, Xuezhao
and me discussed it on communicator and finally figured it out that
it's caused by race between plfs_mkdir and plfs_open/plfs_write.
Basically, plfs_mkdir try to loop all its backend to create directory
but returns if meeting the first error. So the return code of EEXIST
doesn't really mean the directory has been created on all backends.
And later plfs_open/plfs_write may fail because of ENOENT.

We proposed below PLFS patch to ignore EEXIST to make sure when
plfs_mkdir returns, directory has been created on all backends.

This change is made by Zhenhua.