[JFFS2][XATTR] Add a description about c->xattr_sem

Ordering constraints:

	Lock wbuf_sem last, after the alloc_sem or and f->sem.

	c->xattr_sem

	------------

This read/write semaphore protects against concurrent access to the

xattr related objects which include stuff in superblock and ic->xref.

In read-only path, write-semaphore is too much exclusion. It's enough

by read-semaphore. But you must hold write-semaphore when updating,

creating or deleting any xattr related object.

Once xattr_sem released, there would be no assurance for the existence

of those objects. Thus, a series of processes is often required to retry,

when updating such a object is necessary under holding read semaphore.

For example, do_jffs2_getxattr() holds read-semaphore to scan xref and

xdatum at first. But it retries this process with holding write-semaphore

after release read-semaphore, if it's necessary to load name/value pair

from medium.

Ordering constraints:

	Lock xattr_sem last, after the alloc_sem.