inode: don't take lock on whole table during ref/unref

[amarts]

inode_ref/inode_unref are very common operation, and taking
a table->lock makes no complete sense. By making 'ref' as atomic,
we can make most part of the inode_ref/inode_unref as lock-free.

also as a side effect, removed the check for root_inode from
ref/unref functions, thus making them even leaner.

also work on making critical section smaller

• do all the 'static' tasks outside of locked region.
• In this patch, considering hash_dentry() and hash_gfid().
• Also remove extra __dentry_hash exported in libglusterfs.sym

Credits: Mohit Agarwal

updates: #1000
Change-Id: I78f0d77f28ed48cdd0dc7c3489fc8ca32c36e263
Signed-off-by: Amar Tumballi amar@kadalu.io