fs: Fix to correct the mbcache entries counter

This patch fixes the c_entry_count counter of the mbcache. Currently
it increments the counter first & allocate the cache entry later. In
case of failure to allocate the entry due to insufficient memory this
counter is still left incremented. This patch fixes this anomaly.

Signed-off-by: Ram Gupta <ram.gupta5@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>