fs: Handle intra-page faults in copy_mount_options()

The copy_mount_options() function takes a user pointer argument but no
size. It tries to read up to a PAGE_SIZE. However, copy_from_user() is
not guaranteed to return all the accessible bytes if, for example, the
access crosses a page boundary and gets a fault on the second page. To
work around this, the current copy_mount_options() implementation
performs two copy_from_user() passes, first to the end of the current
page and the second to what's left in the subsequent page.

On arm64 with MTE enabled, access to a user page may trigger a fault
after part of the buffer has been copied (when the user pointer tag,
bits 56-59, no longer matches the allocation tag stored in memory).
Allow copy_mount_options() to handle such intra-page faults by returning
-EFAULT only if the first copy_from_user() has not copied any bytes.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>