`seek_mapped` and `seek_unmapped` should hold the mutex
`seek_mapped` and `seek_unmapped` should hold the mutex

Otherwise a parallel thread executing `read` or `write` (which both
involve `seek` internally) might be interfered with.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@unikernel.com>

samoht/mirage-block-unix,djs55/mirage-block-unix,mirage/mirage-block-unix