Linux Compat: inode->i_mutex/i_sem

Linux Compat: inode->i_mutex/i_sem

Create spl_inode_lock/spl_inode_unlock compability macros to simply
access to the inode mutex/sem.  This avoids the need to have to ugly
up the code with the required #define's at every call site.  At the
moment the SPL only uses this in one place but higher layers can
benefit from the macro.