media: vb2: be sure to unlock mutex on errors

As reported by smatch:
drivers/media/common/videobuf2/videobuf2-core.c: drivers/media/common/videobuf2/videobuf2-core.c:2159 vb2_mmap() warn: inconsistent returns 'mutex:&q->mmap_lock'.
  Locked on:   line 2148
  Unlocked on: line 2100
               line 2108
               line 2113
               line 2118
               line 2156
               line 2159

There is one error condition that doesn't unlock a mutex.

Fixes: cd26d1c4 ("media: vb2: vb2_mmap: move lock up")
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>