filemap: filemap_read() should check that the offset is positive or zero

We do check that the read offset is less than the filesystem limit,
however for good measure we should also check that it is positive or
zero, and return EINVAL if that is not the case.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Link: https://lore.kernel.org/r/482ee0b8a30b62324adb9f7c551a99926f037393.1726257832.git.trond.myklebust@hammerspace.com


Signed-off-by: Christian Brauner <brauner@kernel.org>