GFS2: Set gl_object in inode lookup only after block type check

GFS2: Set gl_object in inode lookup only after block type check

Before this patch, the inode glock's gl_object was set after a
reference was acquired, but before the block type was verified.
In cases where the block was unlinked, then freed and reused on
another node, a residule delete callback (delete_work) would try
to look up the inode, eventually failing the block check, but
only after it overwrites gl_object with a pointer to the wrong
inode. This patch moves the assignment of gl_object after the
block check so it won't be improperly overwritten.

Likewise, at the end of the function, gfs2_inode_lookup was
clearing gl_object after it unlocked the glock, which meant
another process might free the glock in the meantime. This
patch guards against that case.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>